Changes in the industry have been dramatic, and many of those changes can be traced back to the advent of advanced simulation software to model the flow of resin within the complex runners, gates and cavities of modern high productivity molds.
it’s simple have never had formal education in plastics fundamentals, because that couldn’t be further from the truth, It’s probably one of the most complex industries, or one of the most complex ways to make a part.
The complexity starts with the unusual properties of thermoplastic resin as a raw material. Engineers and non-engineers alike tend to think of solid substances as materials that melt at a defined temperature, with the melt whose viscosity decreases linearly with temperature. To material scientists, this is called Newtonian material behaviour. While many crystalline materials do behave this way, most commodity thermoplastics simply do not.
If you are specifying or designing a mold, the material properties affect the approach to mold design, and its cost and performance. Add in modern mold accessories such as hot runner systems, and the problems become even more complex, For the better mold designers, that knowledge has to be there. But for many that I’ve interfaced with over the years, I’m not sure that they really do have an understanding of the non-Newtonian flow and what impact some of their decisions have on the part.
While a smooth-running injection molding operation looks as simple as heat, fill, cool and eject, those non-Newtonian material properties must be understood in order to design a productive mold capable of quality part production.
Most mechanical engineers can design fluid power systems with simple assumptions about fluid viscosity and compressibility. However, resin in the melt has flow properties that change with temperature, pressure and shear, meaning the speed and acceleration of the screw matters during mold filling and packing. The length, geometry, surface and cross-sectional area of runners all matter, as does the geometry of cavities.
Mistakes or suboptimal mold design were once common, and press operators learned to alter machine parameters to compensate. Altering temperature was the most common way to get more melt into the cavities faster, but at the risk of degradation of the resin. Much depended on factors outside the tool technology, and good operators were at a premium.